@RobertCohen27
Just to get this right, this issue is whatsapps fault as they do browser sniffing and - again - screwed this up by not detecting Vivaldi correctly.

Just bypass this with a user agent switcher, you can get them via the chrome webstore. Install one of them, click the extension icon and select that vivaldi should cloak itself as chrome --> done.
You maybe need to reload whatsapp afterwards.

5 lines below in the same forum there are sufficient answers and solutions for that

-web.whatsapp.com can no longer be accessed?!
-BUG: WhatsApp Web support is gone in 2.5.1525.40 (Stable channel) (32-bit)
-can't open web whatsapp today

Also, what does the whatsapp message say? : "To use Whatsapp update Chrome or use Firefox, Edge or Opera" Now who killed what? They won't support Vivaldi, Yandex ,Maxthon as well, (when they will be updated to Chromium 74) because they are lazy/ not getting paid for doing so/ being advertised in a sidebar like in Opera, not the opposite.